1. Field of the Invention
The present invention relates to a golf ball which flies a short distance and more particularly, to a golf ball which is prevented from flying a long distance owing to the configuration of dimples formed so that the drag coefficient thereof is high.
2. Description of the Related Art
As is generally known, dimples are formed on a golf ball so as to increase its flight by increasing its lift coefficient and decrease its drag coefficient while the golf ball is flying.
In order to meet golfers' desire to play golf with a golf ball which flies a long distance, researches including the improvement of the configuration of a dimple have been made to provide such a golf ball.
Recently, there is a growing demand for the manufacture of a golf ball which is prevented from flying a long distance so that the following two problems can be solved.
One is that in a small practicing ground, a golf ball driven by a golfer passes through the net of the practicing ground.
The other is that a golfer desires to play golf with a club, capable of flying a golf ball a long distance, such as a driver even in a golf course short from a tee ground to a green.
The following two proposals are made to reduce the flight of a golf ball: According to Japanese Patent Laid-Open Publication No. 60-92780, the mixing ratio of materials of a golf ball is altered to reduce its restitution coefficient. That is, the golf ball is prevented from flying a long distance by reducing its initial speed. According to Japanese Patent Laid-Open Publication No. 61-154683, projections are formed on the surface of a golf ball so as to increase its drag coefficient while it is flying. Thus, it can be prevented from flying a long distance.
The flight of the golf ball in accordance with Japanese Patent Laid-Open Publication No. 60-92780 is approximately 5% shorter than known golf balls. Therefore, the golf ball can be prevented from passing through the net of a practicing ground. But the flying distance of the golf ball is not so short as to allow a golfer to use a driver in a short golf course. Thus, there is room for further researches.
The golf ball having projections formed thereon according to Japanese Patent Laid-Open Publication No. 61-154683 is preferable in that the flying distance of the golf ball is much shorter than known golf balls. But the configurations of dimples are not uniform because of the disadvantage caused by molding of the golf ball and the manufacturing process, which leads to the nonuniform flying performance of the golf ball and the unfine outer appearance.
A golf ball is molded as follows: A male die serving as a master die is manufactured and the male die is necessary for manufacturing semispherical upper and lower female dies. A golf ball is molded by the two female dies. Most of golf balls commercially available have dimples 1A as shown in FIG. 8. A female die 2 having projection 1B as shown in FIG. 9 is required to be manufactured to form the dimples 1A.It is necessary to manufacture a male die 3 having dimples 1C serving as a master die as shown in FIG. 10 in order to manufacture the female die 2. The dimples 1C of the male die 3 having the dimples 1C can be formed with a comparative ease and in a small error by cutting a plain semispherical metallic material with an end mill of an appropriate configuration.
In order to manufacture the golf ball having the dimple 4A, according to Japanese Patent Laid-Open Publication No. 61-154683, as shown in FIG. 11, it is necessary to manufacture a female die 5 having the dimple 4B as shown in FIG. 12. It is necessary to manufacture a male die 6 having projections 4C as shown in FIG. 13 as a master die in order to manufacture the female die 5.
It is necessary to prepare a large-scale electric discharge machining equipment so as to form the male die 6 having the projections 4C. It is expensive to manufacture the male die 6 by the electric discharge machining equipment and further, the configurations of dimples are nonuniform. Consequently, the flight performance of the golf ball having the projections is nonuniform. Further, as described previously, since the golf ball is molded in combination of the semispherical upper and lower dies, a burr is necessarily formed on the seam line between upper and lower dies, namely, on the parting line of the golf ball. In grinding the burr formed on the golf ball having the projections 4A as shown in FIG. 11, the dimples 4A contacts with the grinding stone. Consequently, the burr cannot be abraded sufficiently and dimples adjacent to the parting line are partly worn away, i.e., the dimples are deformed. Accordingly, the golf ball having the projections does not look externally fine and the flight performance thereof is nonuniform.